Abstract

Restoring urban infrastructure and managing the nitrogen cycle represent emerging challenges for urban water quality. We investigated whether stormwater control measures (SCMs), a form of green infrastructure, integrated into restored and degraded urban stream networks can influence watershed nitrogen loads. We hypothesized that hydrologically connected floodplains and SCMs are “hot spots” for nitrogen removal through denitrification because they have ample organic carbon, low dissolved oxygen levels, and extended hydrologic residence times. We tested this hypothesis by comparing nitrogen retention metrics in two urban stream networks (one restored and one urban degraded) that each contain SCMs, and a forested reference watershed at the Baltimore Long-Term Ecological Research site. We used an urban watershed continuum approach which included sampling over both space and time with a combination of: (1) longitudinal reach-scale mass balances of nitrogen and carbon conducted over 2 years during baseflow and storms (n = 24 sampling dates × 15 stream reaches = 360) and (2) 15N push–pull tracer experiments to measure in situ denitrification in SCMs and floodplain features (n = 72). The SCMs consisted of inline wetlands installed below a storm drain outfall at one urban site (restored Spring Branch) and a wetland/wet pond configured in an oxbow design to receive water during high flow events at another highly urbanized site (Gwynns Run). The SCMs significantly decreased total dissolved nitrogen (TDN) concentrations at both sites and significantly increased dissolved organic carbon concentrations at one site. At Spring Branch, TDN retention estimated by mass balance (g/day) was ~150 times higher within the stream network than the SCMs. There were no significant differences between mean in situ denitrification rates between SCMs and hydrologically connected floodplains. Longitudinal N budgets along the stream network showed that hydrologically connected floodplains were important sites for watershed nitrogen retention due to groundwater–surface water interactions. Overall, our results indicate that hydrologic variability can influence nitrogen source/sink dynamics along engineered stream networks. Our analysis also suggests that some major predictors for watershed N retention were: (1) streamwater and groundwater flux through stream restoration or stormwater management controls, (2) hydrologic residence times, and (3) surface area of hydrologically connected features.

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Acknowledgments

This research was supported by MD Sea Grant Awards SA7528085-U, R/WS-2 and NA05OAR4171042, NSF Awards DBI 0640300 and CBET 1058502, EPA NNEMS Award 2010-308, NASA grant NASA NNX11AM28G, the U.S. EPA Office of Research and Development, and Baltimore Ecosystem Study LTER project (NSF DEB-0423476). We thank Melanie Harrison, Jeff Campbell, Katie Delaney-Newcomb, Gwen Sivirichi, Michael Pennino, Dan Dillon, Shuiwang Duan, Casie Smith, and Rich Foot for assistance in the lab and field. Steve Stewart, Prakash Mistry, and Bill Stack provided help with selection of field sites and logistical support. The research has not been subjected to U.S. Environmental Protection Agency review and therefore does not necessarily reflect the views of any of the funding agencies, and no official endorsement should be inferred.

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